How will you spec a small car 4-pot engine?

[dwightlooi]

How will you spec a small car engine -- such as one for the Astra or the Cobalt? If it is up to me it'll be as follows:-

Displacement: 1800 cc

Bore x Stroke: 88 x 74 mm

Construction: Aluminum block and heads.

Harmonic compensation: Twin Lanchester type balancers driven by timing chain.

Compression ratio: 10.8:1 (12.8:1)

Aspiration: Normally aspirated with 4 port mounted intake butterflies and trumpeted velocity stacks.

Valvetrain: Chain driven DOHC 4-valve per cylinder with continuous intake/exhaust VVT

Fuel injection: Direct gasoline injection with 50~150 psi variable pressure rail

Maximum engine speed: 7500 rpm (8,500 rpm)

Maximum piston speed: 3642 fpm (4127 fpm)

Power: 150 hp @ 6800 rpm (200 @ 8300 rpm)

Torque: 130 lb-ft @ 3800 rpm (137 @ 5800 rpm)

Fuel requirement 87 octane (91 octane)

[Z-06]

If you have a Direct Injection system on a smaller mill, I can see it pushing more to about 160-165hp at a compression ratio of about 11.5 (the 3.6DI is 11.3) without sacrificing the fuel economy.

Edited August 20, 2007 by smallchevy

[dwightlooi]

If you have a Direct Injection system on a smaller mill, I can see it pushing more to about 160-165hp at a compression ratio of about 11.5 (the 3.6DI is 11.3) without sacrificing the fuel economy.

I am conservative...

The engine is an oversquare design (88mm x 74 mm) with a significantly larger bore than stroke - the piston speeds it sees at 8500 rpms is roughly the same as that which a 4.2 liter LL8 I6 sees at 6300 rpm. Among other things this tends to lower the compression ratio it can tolerate before pinging becomes an issue. Think of denotation as a race between the spark ignited flame front and the flame front(s) created by hot spots usually in the periphery of the cylinder. Because ignition happens near top dead center bore diameter increments tend to degrade compression tolerance while stroke increases do not -- although both increases displacement. This is because bore increases places the spark plug further away from the furthest reaches of the combustion chamber while stroke increases do not.

The reason you go with an oversquare design is that (1)it reduces piston speeds allowing higher redlines, (2)it reduces 2nd order vibrations allowing for greater refinement especially at higher engine speeds and (3)it allows for larger valve areas per unit engine displacement benefiting breathing again at high RPMs. The penalty is that the engine is more prone to detonation and less tolerant of higher compression ratios, is less efficient at extracting energy from each drop of fuel burned, produces less maximum torque and produces less torque at lower engine speeds.